Just got a '17 LX Manual and I'm seeing the onboard dash calculator show 39.9mpg for a 56 mile round trip of mixed driving where I didn't try to conserve fuel.

I know that is only one trip and will be averaged in with the rest of the driving in this tank of gas (which is now up to over 35mpg). But, how accurate is the on-board dash computer in calculating mpg?

What is a typical difference?

And is the on-board calculator typically optimistic or pessimistic?

 

Can only state my experience, mine seems to be 1 to 2 mpg optimistic compared to doing the actual math....am still pleased with the gas consumption, no matter...

 

Mine seems to be accurate within +/-1 MPG most of the time. It may vary tankful to tankful, but I think over the long run it's pretty good.

 

Mine reads 1-2 MPG higher than my hand calculations.

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Typically overly optimistic by 1 to 3 points. But interestingly I've also seen many times when the dash display was lower than the math, always on interstate runs with only two or three launches per tank. The launches kill you.

 

Yep, consistently comes in about 2 mpg higher than reality.

 

I agree with everyone.
About 1-2 MPG optimistic.
Which in to my knowledge and experience is pretty typical of ALL on dash, built in MPG systems.
I think manufacturers calibrate them to be optimistic.
Hoping we won't calculate manually...and just be that much more happy with the great MPG.

 

The difference from your calculation and what the dash reads could be from the fact that you did not top off the tank. (please don't lecture me on the consequences). For example, if the trip ODO shows 425 miles and you fill the tank to 10.5 gallons, which calculates to 40.48 mpg. Now, if you top the tank, you could possibly fill 11.1 gallons, which calculates 38.29 mpg. You have a difference of 2.2 mpg if you don't top off the tank. To make an accurate calculation, you have to start with a "full" tank and fill it "full" again. If you don't see the gas in the neck of the tank, you cannot assure yourself that you have filled the same amount you used. I have shown an error of 0.6 gallons resulted in an error of 2.2 mpg. My last three fills, I was able to fill slightly more than 11.1 gallons. There is no reason for Honda to toss in a fudge factor in calculating mpg. Also, the speedometer reads accurately when comparing to the speed indicated by my Garmin GPS.

 

This is true here, as well .....guess the "most consistent" way to do the manual calculation is to always use the exact same pump at the same station and not "top it up" after it first clicks off.....not always possible, depending on how and where one uses their car. Car says atm moment I'm getting 37.5 mpg .....if I'm getting 36.5, or even 35.5 jetting back and forth around my city, I'll take it. Mostly country backroads, hills, major roads with lights and occasional hwy (minimal).....mpg goes down when I wind the 6 spd out in the mountains, but....not unexpected

 

I was happy with Fit's MPG no matter how to calculate the MPG compared to my former van's 10-18 MPG.

 

Its overly optimistic. But close enough. Kinda like the gas gauge light with well over 2 gallons in the tank.

 

If you are going to accurately measure mpg, you have to know how much gas you used. There are several factors that would give you a false full tank reading if you use the pump as the indicator. The high flow speed of the gas and the angle of the nozzle in the filler could stop the fill prior to the tank being full. You probable knew that. Now, I started with a full tank, and replenished the tank to full with 11.223 gallons. The trip A ODO indicated I traveled 451.5 miles. If these numbers are accurate, then I received 40.23 mpg. At the same time, the Fit estimated 42.2 mpg, which is 1.97 mpg optimistic as others have indicated similarly. I will compare the calculated vs Fit estimated mpg again after using up the current tank of gas.

 

Right, know that....hence my statement that using the same pump at the same station repeatedly and allowing it to shut off at the same time would help in consistency.....I don't sweat this stuff, am satisfied with the mpg....although hwy mpg could've been much better with a taller 6th gear on the manual, as we all know....I wouldn't mind having to downshift to 5th for hwy acceleration, but...it is what it is, I guess